Compared with super-heterodyne receiver, a zero intermediate-frequency (Zero-IF) receiver down converts radio frequency (RF) signals to baseband directly. It makes the design of a receiver simpler and easier in term of frequency planning. Also it is a cost-efficient solution.
However, due to inherent circuit characteristics of a Zero-IF receiver, it suffers from problems such as DC offset and I/Q mismatch, which would result in degradation of the overall radio performance of the Zero-IF receiver. Currently, a common solution for correction is to add a digital engine in a receiver after analog to digital (AD) conversion of received signals so as to correct signal errors induced by the DC offset and I/Q mismatch, which, however, requires training a correction engine with a training signal in accordance with a correction algorithm.
In an existing receiver of a wireless communication system, a common method is to adopt a blind estimation in a correction process (i.e. a training signal to be used is unknown in advance). Taking a receiver in a base station as an example, such receiver generally employs a received user equipment (UE) signal as training signal. The power of a received UE signal is unpredictable. It may be very low or very high, or may change very fast; and in an extreme situation, there may be even no UE in the network. Thus, a correction made based on such training signals may not converge. Moreover, the correction quality is heavily dependent on the statistic characteristics of received UE signals. However, the number of UEs always dynamically varies, and the received UE signals are actually mixed signals from different UEs, thus the statistic characteristic of the received UE signals may change greatly, which would result in degradation of the correction quality in a Zero-IF receiver.
Since incoming signals to be received are difficult to predict, a correction made based on such training signals would lead to unstable performance of a correction engine or increased complexity of a correction algorithm in a wireless communication system. Meanwhile, correction should be a real-time process in consideration of the ageing problem. Accordingly, in many cases compromises have to be made between algorithm performance and complexity.
The above problems prevent Zero-IF receivers from being widely used in a wireless communication system.